Disengagement detecting device and method

ABSTRACT

The present invention relates to a disengagement detection device and method capable of detecting the disengagement of a wheel from a vehicle easily and reliably. A magnetic sensor  11  detects the magnetism naturally accumulated in a space inside a vehicle at or near a wheel mounted on the vehicle. A control unit  21  determines, for each prescribed interval, the average value of the output sensor data based on the magnetism detected with the magnetic sensor  11 , judges that the wheel disengaged from the vehicle when the average value of the sensor data fluctuates by a prescribed threshold value or more and thereafter stabilizes at a value that is not in a stationary state, and controls the output unit  13  so as to output a disengagement notification signal to the outside. The present invention may be employed in antitheft devices for car wheels.

TECHNICAL FIELD

The present invention generally relates to a disengagement detectiondevice and method, and particularly to a disengagement detection deviceand method capable of detecting the disengagement of a wheel from avehicle easily and reliably.

BACKGROUND ART

In recent years, incidents where wheels in a state with tires fittedthereon being stolen are increasing, and sensors for preventing suchtheft are commercially available. As such antitheft sensor, there is atype that uses an inclination sensor for detecting the tilting of thevehicle to detect the theft of a wheel by detecting the tilting of thevehicle when it is jacked up by the thief stealing the wheel.

Further, a method has been proposed for installing a contact sensor tothe vehicle so as to contact the surface of the tire while the car isparked and detecting the theft of a tire by detecting that the tire hasbeen disengaged from the contact sensor, as well as detecting that thecontact sensor itself has been moved in order to prevent the tire frombeing stolen in a state with the contact sensor being in contact withthe tire (e.g., Patent Document 1).

[Patent Document 1] Japanese Patent Laid-Open Publication No.2002-037029

DISCLOSURE OF THE INVENTION

Nevertheless, in recent years, the tactics used by thieves are becomingastute, and there are many cases where wheels are stolen by raising, butnot tilting, the vehicle such as by placing blocks or bricks under thevehicle and thereafter deflating the tires, or simultaneously jacking upfour locations near the tires of the vehicle. In these cases, it isdifficult for conventional antitheft sensors employing an inclinationsensor to detect the theft of wheels.

Further, with the invention described in Patent Document 1, theantitheft sensor needs to be mounted on the vehicle each time the car isparked, and this is an extremely troublesome task for the user.Moreover, since the antitheft sensor is exposed outside in a state ofbeing mounted on the vehicle, there is a problem in that it may bedestroyed to prevent such sensor from operating.

The present invention was devised in view of the foregoingcircumstances, and an object thereof is to enable the detection of thedisengagement of a wheel from a vehicle easily and reliably.

The first disengagement detection device according to the presentinvention comprises: a detection means for detecting the magnetisminside the vehicle at or near the wheel mounted on the vehicle; and ajudgment means for judging the disengagement of the wheel from thevehicle based on whether the value of the magnetism detected with thedetection means changed by a prescribed threshold value or more, andwhether a first stable state centered around a first value changed to asecond stable state centered around a second value.

With the first disengagement detection device of the present invention,the detection means detects the magnetism inside the vehicle at or nearthe wheel mounted on the vehicle; and the judgment means judges thedisengagement of the wheel from the vehicle based on whether the valueof the magnetism detected with the detection means changed by aprescribed threshold value or more, and whether a first stable statecentered around a first value changed to a second stable state centeredaround a second value.

Therefore, the disengagement of the wheel from the vehicle can bedetected easily and reliably.

The detection means, for example, is constituted from a highly sensitivemagnetic sensor capable of detecting earth magnetism, and the judgmentmeans is constituted from a judgment unit for judging the disengagementof the wheel from the vehicle based on whether the magnetism inside thevehicle at or near the wheel changed significantly in comparison to thewidth of change in a stable state, and whether it became stable in suchchanged state.

The detection means may detect magnetisms in a plurality of differentdirections, and the judgment means may judge the disengagement of thewheel from the vehicle based on the comparison of the value of themagnetisms detected with the detection means, and the distribution ofthe value of the magnetisms detected previously with the detection meansin a state where the wheel is mounted on the vehicle.

As a result, the disengagement of the wheel from the vehicle can bedetected easily and reliably.

The second disengagement detection device according to the presentinvention comprises: a detection means for detecting magnetisms in aplurality of different directions inside the vehicle at or near thewheel mounted on the vehicle; and a judgment means for judging thedisengagement of the wheel from the vehicle based on the comparison ofthe value of the magnetisms detected with the detection means, and thedistribution of the value of the magnetisms detected previously with thedetection means in a state where the wheel is mounted on the vehicle.

With the second disengagement detection device of the present invention,the detection means detects magnetisms in a plurality of differentdirections inside the vehicle at or near the wheel mounted on thevehicle; and the judgment means judges the disengagement of the wheelfrom the vehicle based on the comparison of the value of the magnetismsdetected with the detection means, and the distribution of the value ofthe magnetisms detected previously with the detection means in a statewhere the wheel is mounted on the vehicle.

Therefore, the disengagement of the wheel from the vehicle can bedetected easily and reliably.

The detection means, for example, is constituted from a highly sensitivemagnetic sensor capable of detecting earth magnetism, and the judgmentmeans is constituted from a judgment unit for judging the disengagementof the wheel from the vehicle based on whether the magnetism inside thevehicle at or near the wheel became a value different from thedistribution in a state where the wheel is mounted.

The first disengagement detection device and second disengagementdetection device of the present invention may further comprise anotification means for notifying that the disengagement of the wheelfrom the vehicle has been detected.

As a result, the disengagement of the wheel from the vehicle detectedwith the first disengagement detection device and second disengagementdetection device can be notified outside.

The notification means, for example, is constituted from a radio.

The first disengagement detection method according to the presentinvention for detecting the disengagement of a wheel from a vehiclecomprises: a detection step for detecting the magnetism inside thevehicle at or near the wheel mounted on the vehicle; and a judgment stepfor judging the disengagement of the wheel from the vehicle based onwhether the value of the magnetism detected by the processing in thedetection step changed by a prescribed threshold value or more, andwhether a first stable state centered around a first value changed to asecond stable state centered around a second value.

This detection step, for example, is constituted from a detection stepfor acquiring sensor data based on the magnetism inside the vehicle ator near the wheel mounted on the vehicle detected with the magneticsensor, and the judgment step, for instance, is constituted from ajudgment step for judging the disengagement of the wheel from thevehicle based on whether the value of the acquired sensor data changedsignificantly in comparison to the width of change in a state where themagnetism is stable, and whether it become state in such changed value.

The second disengagement detection method according to the presentinvention for detecting the disengagement of a wheel from a vehiclecomprises: a detection step for detecting magnetisms in a plurality ofdifferent directions inside the vehicle at or near the wheel mounted onthe vehicle; and a judgment step for judging the disengagement of thewheel from the vehicle based on the comparison of the value of themagnetisms detected by the processing in the detection step, and thedistribution of the value of the magnetisms detected previously by theprocessing in the detection step in a state where the wheel is mountedon the vehicle.

This detection step, for example, is constituted from a detection stepfor acquiring sensor data based on the magnetism inside the vehicle ator near the wheel mounted on the vehicle detected with the magneticsensor, and the judgment step, for instance, is constituted from ajudgment step for judging the disengagement of the wheel from thevehicle based on whether the value of the acquired sensor data became avalue different from the distribution in a state where the wheel ismounted.

According to the present invention, the disengagement of the wheel fromthe vehicle can be detected. Particularly, according to the presentinvention, the disengagement of the wheel from the vehicle can bedetected easily and reliably without requiring the user to engage inspecial operations on a case-by-case basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the hardware configuration of anembodiment of the disengagement detection device employing the presentinvention;

FIG. 2 is a block diagram showing a functional configuration example ofthe control unit illustrated in FIG. 1;

FIG. 3 is a diagram showing an installation example of the disengagementdetection device illustrated in FIG. 1;

FIG. 4 is a diagram showing an example of the sensor data to be outputfrom the magnetic sensor;

FIG. 5 is a diagram showing an example of the distribution of the sensordata to be output from the magnetic sensor;

FIG. 6 is a diagram showing an example of the distribution of the sensordata to be output from the magnetic sensor;

FIG. 7 is a flowchart for explaining the disengagement detectionprocessing based on the fluctuation detection system in thedisengagement detection device illustrated in FIG. 1;

FIG. 8 is a flowchart for explaining the disengagement detectionprocessing based on the fluctuation detection system in thedisengagement detection device illustrated in FIG. 1;

FIG. 9 is a block diagram showing the hardware configuration of anembodiment of the disengagement detection device employing the presentinvention;

FIG. 10 is a block diagram showing a functional configuration example ofthe control unit illustrated in FIG. 9;

FIG. 11 is a flowchart for explaining the disengagement detectionprocessing based on the distribution data detection system in thedisengagement detection device illustrated in FIG. 9;

FIG. 12 is a flowchart for explaining the disengagement detectionprocessing based on the distribution data detection system in thedisengagement detection device illustrated in FIG. 9;

FIG. 13 is a flowchart for explaining the disengagement detectionprocessing based on the combination detection system in thedisengagement detection device illustrated in FIG. 9; and

FIG. 14 is a flowchart for explaining the disengagement detectionprocessing based on the combination detection system in thedisengagement detection device illustrated in FIG. 9.

LIST OF ELEMENTS

-   -   1 disengagement detection device    -   11 magnetic sensor    -   12 microcomputer    -   13 output unit    -   14 input unit    -   21 control unit    -   22 memory unit    -   23 ROM    -   31 data processing unit    -   32 judgment unit    -   41 tire housing    -   42 floor    -   43 axle    -   101 disengagement detection device    -   112 microcomputer    -   114 input unit    -   121 control unit    -   131 learning processing unit    -   132 judgment unit

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are now explained with reference tothe drawings.

FIG. 1 is a diagram representing the hardware configuration of adisengagement detection device 1 employing the present invention.

The disengagement detection device 1 is constituted from a magneticsensor 11, a microcomputer 12, an output unit 13, and an input unit 14.

The magnetic sensor 11, for example, has a built-in MI (MagnetImpedance) element, and detects magnetism. The MI element is used fordetecting magnetism by utilizing the MI effect in which the magnetismimpedance changes significantly based on the strength of the outsidemagnetic field when a high frequency current is applied to a magneticsensitive medium such as amorphous, and it is also capable of detectingearth magnetism (i.e., it is capable of detecting the absolute bearing).The magnetic sensor 11 comprises two such highly sensitive sensors, andthe respective sensors detect magnetism in the mutually intersectingbiaxial direction (hereinafter, one direction is referred to as the xaxis direction, and the other direction is referred to as the y axisdirection).

The magnetic sensor 11 detects the biaxial direction magnetism as thesize of the magnetic flux density with milligauss as the unit thereof.Therefore, in the example of the sensor data output from the magneticsensor 11 illustrated later in FIG. 4 to FIG. 6, the value of the sensordata is the value of the magnetic flux density with milligauss as theunit thereof. Needless to say, the magnetic sensor 11 may also detectthe biaxial direction magnetism as the strength of the magnetic fieldwith ampere per meter or Newton per Weber as the unit thereof. Further,the magnetic sensor 11 may also detect magnetism in triaxial directionsor more.

The magnetic sensor 11, as described later with reference to FIG. 3, isinstalled inside the vehicle adjacent to the tire housing that isroughly directly overhead the axle, and detects the magnetism of thespace inside the vehicle that is naturally accumulating at and near thewheel (hereinafter referred to as the wheel vicinity) in vehicle. Themagnetic sensor 11 supplies to a microcomputer 12 the sensor data basedon the detected magnetism.

The microcomputer 12, as described later with reference to FIG. 7 andFIG. 8, detects the disengagement of a wheel by detecting the change ofmagnetism of the wheel vicinity based on the sensor data supplied fromthe magnetic sensor 11. When the microcomputer 12 detects thedisengagement of a wheel, it controls the output unit 13 and outputs adisengagement notification signal for notifying the disengagement of thewheel.

The output unit 13 outputs via wireless transmission the disengagementnotification signal outside the disengagement detection device 1 basedon the control from the microcomputer 12. The output unit 13 may, inaddition to transmitting the disengagement notification signal, emittinga warning sound or light for notifying the disengagement of the wheel tothe outside.

The input unit 14 is to be operated by a user, and it outputs thecommand from the user to the microcomputer 12. For example, the user mayoperate the input unit 14 to command the ON or OFF of the wheeldisengagement detection processing (hereinafter simply referred to asthe disengagement detection processing) so as to start the disengagementdetection processing, or stop the disengagement detection processingwhen it is not necessary to perform this disengagement detectionprocessing such as while driving or changing wheels. For instance, aswitch or the like may be provided as the input unit 14, and, by turningsuch switch ON and OFF, the ON and OFF of processing may be outputthereby.

The microcomputer 12 has a control unit 21, a memory unit 22, and a ROM23.

The control unit 21 constituted from the likes of a CPU (CentralProcessing Unit) and RAM (Random Access Memory), as described later withreference to FIG. 7 and FIG. 8, performs the wheel disengagementdetection processing based on a program or the like read out from a ROM23. The control unit 21 stores the sensor data supplied from themagnetic sensor 11 in a memory unit 22 constituted from the likes of asemiconductor memory, and, based on the sensor data stored in the memoryunit 22, stores in the memory unit 22 the average value of the sensordata sought in each prescribed interval (e.g., 1 second). The controlunit 21 detects the disengagement of the wheel by detecting the changeof magnetism in the wheel vicinity based on the average value of thesensor data. The control unit 21 outputs the detection result to theoutput unit 13 when it detects the disengagement of the wheel.

The ROM 23 stores programs and data prestored as the factory default,and supplies the requested program or data to the control unit 21.

FIG. 2 is a block diagram showing a functional configuration example ofthe control unit 21.

The control unit 21 is constituted from a data processing unit 31, ajudgment unit 32, and a command unit 33.

The data processing unit 31 stores the sensor data supplied from themagnetic sensor 11 in the memory unit 22, determines the average valueof the sensor data for each prescribed interval (e.g., 1 second) basedon the sensor data stored in the memory unit 22, and supplies this tothe judgment unit 32.

The judgment unit 32 stores the average value of the sensor datasupplied from the data processing unit 31 in the memory unit 22. Thejudgment unit 32, after the disengagement detection processing isstarted, determines the value of the sensor data (hereinafter referredto as the sensor data reference value) in a state where the averagevalue of the sensor data output from the magnetic sensor 11 is in astable state (hereinafter referred to as the stationary state). Thejudgment unit 32, as described later with reference to FIG. 7 and FIG.8, judges whether the wheel has disengaged by detecting whether themagnetism in the wheel vicinity changed from a stationary state based onthe variation in the average value of the sensor data, and thedifference between the average value of the sensor data and the sensordata reference value. The judgment unit 32 outputs the detection resultto the output unit 13 when it judges that the wheel disengaged. Further,[the judgment unit 32] also manages the reference value setting flagwhich represents whether the sensor data reference value has been set ornot. Further, the judgment unit 32 has a built-in timer (not shown), andmeasures the output stability time and abnormality duration describedlater.

The command unit 33 acquires a command from the user via the input unit14, and controls the data processing unit 31 and judgment unit 32 basedon such command.

FIG. 3 is a diagram showing an example of the installation position ofthe magnetic sensor 11. FIG. 3 is a plan view of the left side in thetrunk room at the rear of the vehicle. A half-column tire housing 41 isprotruding from the left end side of the floor 42 inside the trunk room,and, below the tire housing 41, a left rear tire not shown is mounted onthe vehicle in a state where the wheel is fitted therein. Below thefloor 42, an axle 43 to which the wheel (tire) not shown is mounted ispassing through from near the center of the right end of the tirehousing 41 in the horizontal rightward direction.

The magnetic sensor 11, for example, as shown in FIG. 3, is installed ata position above the floor adjacent near the center of the right endside of the tire housing 41. In the example illustrated in FIG. 3, themagnetic sensor 11, as shown with the arrows, is installed in a plane(horizontal plane) that is parallel with the floor 42 (ground), androughly perpendicular to the x axis direction to become the forwarddirection of the vehicle and the y axis direction, so as to detect thebiaxial direction magnetism of the y axis direction oriented toward theleft side of the vehicle. The magnetic sensor 11 is fixed to the floor42 of the trunk room with a double-sided tape or face fastener.

The magnetic sensor 11 is desirably installed to a position near thetire housing 41 (wheel and tire) as much as possible and immediatelyabove the axle 43 passing below the floor 42 in order to reliably detectthe change of magnetism naturally accumulated in the wheel and tire.Although an aluminum wheel will not be magnetized, certain aluminumwheels have parts constituted from an injection valve or other metalsthat will be magnetized, and the wire built in the tire is alsoconstituted from a material that will be magnetized. The axle 43 willalso be magnetized. Since the magnetism obtained comprehensively by suchmagnetization will be detected, it is desirable that the magnetic sensor11 is disposed in this position. Further, the magnetic sensor 11 fordetecting the disengagement of wheels in other positions of the vehicleis preferably installed at similar positions

As described above, by installing the magnetic sensor 11 inside thevehicle, it is possible to prevent the magnetic sensor 11 from beingdestroyed or removed. Further, when the disengagement detection device 1containing the magnetic sensor 11, microcomputer 12, output unit 13, andinput unit 14 is to be constituted integrally, wiring will not benecessary, and the disengagement detection device 1 itself can be easilyinstalled inside the vehicle merely by disposing it near the right sideend of the tire housing 41. Incidentally, in the following explanation,the disengagement detection device 1 is explained as integrallyconstituting the magnetic sensor 11, microcomputer 12, output unit 13,and input unit 14.

FIG. 4 is a diagram showing an example of the sensor data output fromthe sensor for detecting the magnetism in the x axis direction of themagnetic sensor 11 when the wheel is removed from the vehicle in a statewhere the disengagement detection device containing the magnetic sensor11 is disposed at the position illustrated in FIG. 3. In FIG. 4, thehorizontal axis represents time, and the vertical axis represents thevalue of sensor data (unit: milligauss). The wheel is removed from thevehicle during the period represented with section T0 in FIG. 4.

As shown in FIG. 4, in section T1 before section T0 where the wheel isremoved, the value of sensor data is in a stable state (stationarystate) roughly at M1. In other words, in this state, the variation widthof the value of sensor data is small. Meanwhile, in section T2 aftersection T0 where the wheel is removed, the sensor data is in a stablestate at value M2 that is different from value M1. In other words,before the wheel is removed, the magnetism in the wheel vicinity thatwas stable at the first value M1 changes significantly in comparison tothe variation width in a stationary state as a result of the wheel beingremoved, and becomes stable at the second value M2 that is differentfrom the state before the wheel was removed.

FIG. 5 is a diagram showing, in a state where the disengagementdetection device 1 containing the magnetic sensor 11 is disposed at theposition illustrated in FIG. 3, an example of the distribution of thesensor data output from the magnetic sensor 11 when an aluminum wheel ismounted on the vehicle and the tire (aluminum wheel) is rotated once,and the value of sensor data output from the magnetic sensor 11 in astate where the aluminum wheel is removed from the vehicle. In FIG. 5,the horizontal axis represents the value of sensor data (unit:milligauss) in the x axis direction of the magnetic sensor 11, and thevertical axis represents the value of sensor data (unit: milligauss) inthe y axis direction of the magnetic sensor 11. The curve 51 plots onthe graph of FIG. 5 the result of continuously measuring the value ofthe x axis direction and y axis direction of the sensor data output fromthe magnetic sensor 11 while rotating the tire (aluminum wheel) once.The point 52 plots on the graph of FIG. 5 the value of the x axisdirection and y axis direction of the sensor data output from themagnetic sensor 11 in a state where the aluminum wheel is removed.

The curve 51 continuously changes pursuant to the rotation of the tire(aluminum wheel), returns to its original position for each rotation,and becomes a closed curve formed from inner and outer double circles.In other words, when the vehicle is stopped, the value of sensor datawill become a value corresponding to one of the points on the curve 51according to the relative position of the tire (aluminum wheel) inrelation to the vehicle.

Meanwhile, when the aluminum wheel is removed, the value detected withthe magnetic sensor 11 with be a value represented with the point 52(that is discontinuous with the curve 51) separated a certain distanceor more from any point on the curve 51. In other words, regardless ofthe rotating position of the aluminum wheel, when the aluminum wheel isremoved, the magnetism in the wheel vicinity detected with the magneticsensor 11 will change by a certain value or more, and the value aftersuch change will become a value that is different from the value(distribution) at the time the aluminum wheel was installed.

FIG. 6 is a diagram showing, in a state where the disengagementdetection device 1 containing the magnetic sensor 11 is disposed at theposition illustrated in FIG. 3 upon installing a steel wheel instead ofan aluminum wheel to a different vehicle from the one where the sensordata was measured as shown in FIG. 5, an example of the distribution ofthe sensor data output from the magnetic sensor 11 when an aluminumwheel is mounted on the vehicle and the tire (steel wheel) is rotatedonce, and the value of sensor data output from the magnetic sensor 11 ina state where the steel wheel is removed from the vehicle.

As with FIG. 5, the curve 61 represents the distribution of the sensordata output from the magnetic sensor 11 when an aluminum wheel ismounted on the vehicle and the tire (steel wheel) is rotated once, andthe point 62 represents the value of sensor data output from themagnetic sensor 11 in a state where the steel wheel is removed from thevehicle. In the case of a steel wheel, as with the case of an aluminumwheel, the curve 61 continuously changes pursuant to the rotation of thetire (steel wheel), returns to its original position for each rotation,and becomes a closed curve formed from inner and outer double circles.Further, in the case of a steel wheel, in comparison to the case of analuminum wheel, the distance between the curve 61 and point 62 is far.In other words, the change in value of the sensor data as a result ofremoving the steel wheel is greater.

As described with reference to FIG. 4 to FIG. 6, the sensor data outputfrom the magnetic sensor 11 will change significantly in comparison tothe fluctuation range in a stationary state, regardless of the rotatingposition of the wheel, when the wheel is removed from the vehicle(magnetism in the wheel vicinity changes significantly in comparison tothe change in a stationary state), and becomes stable at the value afterfluctuation. The disengagement detection device 1 detects thedisengagement of the wheel from the vehicle by detecting the change ofmagnetism in the wheel vicinity before and after the wheel is removed.This detection system is hereinafter referred to as the fluctuationdetection system.

Incidentally, since the disengagement detection device 1 detects thedisengagement of the wheel based on the magnetism naturally accumulatedin the wheel vicinity, no special operation such as the user activelymagnetizing the wheel with a magnet for detecting the disengagement ofthe wheel will be required.

Next, the disengagement detection processing of the fluctuationdetection system to be executed with the disengagement detection device1 is explained with reference to the flowcharts illustrated in FIG. 7and FIG. 8. Incidentally, this processing is started when the usercommands the start of the disengagement detection processing via theinput unit 14.

At step 1, the initialization processing is executed. Specifically, thejudgment unit 32 turns OFF the reference value setting flag representingwhether the sensor data reference value has been set, activates thetimer, and starts measuring the output stability time. The judgment unit32 measures the time where the variation in the average value of sensordata continues to be stable in a state less than a prescribed thresholdvalue (hereinafter referred to as the fluctuation threshold value).

At step S2, the data processing unit 31 acquires the sensor data basedon the magnetism in the wheel vicinity detected with the magnetic sensor11 from the magnetic sensor 11, and stores this in the memory unit 22.

At step S3, the data processing unit 31 determines the average value ofthe sensor data (sensor data for 1 second) stored in the memory unit 22for each prescribed period (e.g., 1 second), and outputs such value tothe judgment unit 32. The judgment unit 32 stores the average value ofthe output sensor data in the memory unit 22.

At step S4, the judgment unit 32 judges whether the reference valuesetting flag is ON. Here, since the reference value setting flag is OFFin the initialization processing at step S1, the processing proceeds tostep S5.

At step S5, the judgment unit 32 compares the average value of thelatest sensor data computed with the processing at step S3 and theaverage value of the sensor data computed with the processing at theprevious step S3, and judges whether the average value of the sensordata in at least one direction among the x axis direction and y axisdirection has fluctuated by the preset fluctuation threshold value ormore. Here, since the first processing at step S3 was just performed andthe average value of the previous sensor data has not yet been stored inthe memory unit 22, it will be judged that the average value of thesensor data has not fluctuated by the fluctuation threshold value ormore, and the processing will proceed to step S6. Incidentally, at stepS5, as a result of comparing the average value of the sensor data,malfunctions in the value of sensor data caused by the noise componentsfluctuating instantaneously can be suppressed. Therefore, when there isnot much noise, the average value computation processing at step S53 maybe omitted.

At step S6, the judgment unit 32 judges whether the output stabilitytime being measured has exceeded a prescribed period (e.g., 30 seconds)(hereinafter referred to as the reference value measurement period).Here, since the processing has just begun, it will be judged that theoutput stability time has not exceeded the reference value measurementperiod, the processing of steps S7 and S8 will be skipped, and theprocessing will proceed to step S10.

At step S10, the command unit 33 determines whether to end thedisengagement detection processing; that is, it determines whether theuser commanded the end of the disengagement detection processing via theinput unit 14. When the user commands the end of the disengagementdetection processing, the disengagement detection processing is ended,and, when the user did not command the end of the disengagementdetection processing, the processing returns to step S2. In thefollowing explanation, unless specifically indicated, a case isexplained where the user did not command the end of the disengagementdetection processing at step S10, and the processing returns to step S2.

Thereafter, at step S6, the foregoing processing is repeated until theoutput stability time is judged as exceeding the reference valuemeasurement period; that is, until the period where the average value ofthe sensor data fluctuates stably within a range less than thefluctuation threshold value continues during the reference valuemeasurement period. Nevertheless, at step S5, when the average value ofthe sensor data in at least one direction among the x axis direction andy axis direction is judged as fluctuating by the fluctuation thresholdvalue or more, the processing proceeds to step S9, and the judgment unit32 resets the timer, and starts remeasuring the output stability time.

At step S6, when it is judged that the output stability time exceeds thereference value measurement period, the processing proceeds to step S7,and the judgment unit 32 sets the sensor data reference value. Thejudgment unit 32 determines the further average value (average value inthe reference value measurement period) of the average value of thesensor data stored in the memory unit 22 during the reference valuemeasurement period in which the average value of the sensor data becamestable, and sets such value to the sensor data reference value. In otherwords, the sensor data reference value is the average value (value M1)of the sensor data output from the magnetic sensor 11 in section T1 of astationary state before section T0 where the wheel is removed as shownin the graph of FIG. 4.

At step S8, the judgment unit 32 turns ON the reference value settingflag representing that the sensor data reference value has been set,stops the timer, and stops the measurement of the output stability time.Thereafter, the processing proceeds to step S10, then returns to stepS2.

After the sensor data reference value is set, pursuant to the processingat steps S2 and S3, after the average value of the sensor data is outputfrom the data processing unit 31 to the judgment unit 32 based on thesensor data output from the magnetic sensor 11, at step S4, it is judgedthat the reference value setting flag is ON, and the processing proceedsto step S11.

At step S11, the judgment unit 32 judges whether the average value ofthe latest sensor data computed with the processing at step S3 is avalue within a prescribed range (M1±A) centered around the sensor datareference value with respect to both directions of the x axis directionand y axis direction (whether it is in the first stationary state withinthe width A centered around value M1). When the average value of thesensor data is a value within a prescribed range centered around thesensor data reference value with respect to both directions of the xaxis direction and y axis direction, it is judged that the average valueof the sensor data (magnetism in the wheel vicinity) is in a stationarystate, and the processing proceeds to step S18.

At step S18, the judgment unit 32, in the case of measuring theabnormality duration described later, stops the timer and stops themeasurement of the abnormality duration. Thereafter, the processingproceeds to step S10, returns to step S2, and the subsequent processingsteps are repeated.

At step S11, when it is judged that the average value of the sensor datain at least one direction among the x axis direction and y axisdirection is a value outside the prescribed range centered around thesensor data reference value, the average value of the sensor data(magnetism in the wheel vicinity) will not be in a stationary state, andthe processing proceeds to step S12.

At step S12, the judgment unit 32 compares the average value of thelatest sensor data and the average value of the previous sensor datastored in the memory unit 22, and judges whether the average value ofthe sensor data of at least one direction among the x axis direction andy axis direction fluctuated by the preset fluctuation threshold value ormore. This fluctuation threshold value, for instance, may be a valuethat is 0.8 times the absolute value of the difference of values M1 andM2 (0.8×|M1−M2|). When it is judged that the average value V of thesensor data of at least one direction among the x axis direction and yaxis direction fluctuates by the fluctuation threshold value or more;that is, when it is a value outside the prescribed range (not astationary state) centered around the sensor data reference value (avalue smaller than M1−A or greater than M1+A), and the average value Vof the sensor data fluctuates by the fluctuation threshold value or more(fluctuation range (|M1−V|) is greater than (0.8×|M1−M2|)), theprocessing proceeds to step S17.

At step S17, the judgment unit 32 activates the timer, and startsmeasuring the abnormality duration. The judgment unit 32, based on thisabnormality duration, measures the time where, while the average valueof the sensor data is not in a stationary state, the variation in theaverage value of the sensor data continues to be in a stable state lessthan the fluctuation threshold value (second stationary state within therange of width B (fluctuation threshold value) centered around valueM2); that is, the time the magnetism in the wheel vicinity becomesstable in a non-stationary state. Thereafter, the processing proceeds tostep S10, returns to step S2, and the subsequent processing steps arerepeated.

At step S12, when it is judged that neither average value of the sensordata of both directions of the x axis direction and y axis direction hasfluctuated by the fluctuation threshold value or more; that is, when theaverage value V of the sensor data is a value outside the prescribedrange (M1±A) centered around the sensor data reference value M1, but hasnot fluctuated by the fluctuation threshold value or more (when thefluctuation range (|M1−V|) is smaller than (0.8×|M1−M2|)), theprocessing proceeds to step S13.

At step S13, the judgment unit 32 judges whether the abnormalityduration exceeded a prescribed period (hereinafter referred to as thedisengagement detection period). When the measurement of the abnormalityduration has not started, or when the disengagement detection period hasnot elapsed from the point when the measurement of abnormality durationwas started, it is judged that the abnormality duration has not exceededthe disengagement detection period, and the processing at steps S14 toS16 is skipped, the processing proceeds to step S110, thereafter returnsto step S2, and the subsequent processing steps are repeated.

At step S13, when it is judged that the abnormality duration exceededthe disengagement detection period; that is, when the average value V ofthe sensor data of at least one direction among the x axis direction andy axis direction fluctuates by the fluctuation threshold value(0.8×|M1−M2|) or more, becomes a value outside the prescribed rangecentered around the sensor data reference value, and, while remaining atthe value outside the prescribed range centered around the sensor datareference value, and a stable period in which the fluctuation range ofthe average value is less than the fluctuation threshold value (M2±B)continues during the disengagement detection period, for instance, asshown with section T2 after section T0 in FIG. 4, the magnetism in thewheel vicinity will change from M1 to M2 due to the disengagement of thewheel, and, when it is judges as being stable at value M2, theprocessing proceeds to step S14.

At step S14, the judgment unit 32 outputs a disengagement notificationsignal outside the disengagement detection device 1 via the output unit13 (for notifying the user that the wheel has been stolen).

At step S15, the judgment unit 32 turns OFF the reference value settingflag, and, at step S16, the judgment unit 32 stops the measurement ofthe abnormality duration, and starts the measurement of the outputstability time. Thereafter, the processing proceeds to step S10,thereafter returns to step S2, and the sensor data reference valuesetting processing is performed once again.

Incidentally, during the measurement of the abnormality duration, atstep S12, when it is judged that the average value of the sensordirection of at least one direction among the x axis direction and yaxis direction fluctuated by the fluctuation threshold value or more;that is, when the average value of the sensor data is fluctuatingunstably in short cycles in a value outside the prescribed rangecentered around the sensor data reference value, it is judged that themagnetism in the wheel vicinity is unstable due to reasons (e.g.,rotation of the tire) other than the disengagement of the wheel, and theprocessing proceeds to step S117, the timer is reset, and theremeasurement of the abnormality duration is started.

As described above, the magnetism in the wheel vicinity changingsignificantly in comparison to the stationary state due to thedisengagement of the wheel based on the sensor data output from themagnetic sensor 11, and the disengagement of the wheel is detected bydetecting that such changed state became stable.

Meanwhile, as shown in FIG. 5 and FIG. 6, the value of sensor dataoutput from the magnetic sensor 11 changes periodically pursuant to therotation of the tire (wheel) and represents a specific distribution onthe one hand, and, when the wheel is removed from the vehicle, this willbecome a value that is different from the distribution of the value ofsensor data in a state where the wheel is mounted on the vehicle. As aresult of utilizing this characteristic, the disengagement of the wheelcan also be detected by creating distribution data representing thedistribution of the value of sensor data in a state where the wheel ismounted on the vehicle (hereinafter simply referred to as thedistribution data), and comparing the value of sensor data output fromthe magnetic sensor 11 and the created distribution data. This detectionsystem is hereinafter referred to as the distribution data detectionsystem.

FIG. 9 is a diagram representing hardware configuration example of thedisengagement detection device 101 employing the distribution datadetection system. Incidentally, the components common with thedisengagement detection device 1 of FIG. 1 given the same referencenumeral, and redundant explanations will be omitted.

The disengagement detection device 101 is constituted from a magneticsensor 11, a microcomputer 112, an output unit 13, and an input unit114.

The microcomputer 112, as described later with reference to FIG. 11 andFIG. 12, creates distribution data based on the sensor data suppliedfrom the magnetic sensor 11. And, the microcomputer 112 detects thedisengagement of the wheel by comparing the sensor data supplied fromthe magnetic sensor 11 and the created distribution data. Themicrocomputer 112, upon detecting the disengagement of the wheel,controls the output unit 13 so as to output a disengagement notificationsignal notifying the disengagement of the wheel.

The input unit 114 is to be operated by a user, and it outputs thecommand from the user to the microcomputer 112. For example, the usermay operate the input unit 114 to command the ON or OFF of thedisengagement detection processing so as to start the disengagementdetection processing, or stop the disengagement detection processingwhen it is not necessary to perform this disengagement detectionprocessing such as while driving or changing wheels. Further, the usermay operate the input unit 114 and command the renewal of thedistribution data created previously. For instance, a switch or the likemay be provided as the input unit 114, and, by turning such switch ONand OFF, the ON and OFF of processing may be output, or the command forrenewing the distribution data may be output thereby.

The microcomputer 112 has a control unit 121, a memory unit 22, and aROM 23.

The control unit 121 constituted from the likes of a CPU (CentralProcessing Unit) and RAM (Random Access Memory), as described later withreference to FIG. 11 and FIG. 12, performs the wheel disengagementdetection processing based on a program or the like read out from a ROM23. The control unit 121 stores the sensor data supplied from themagnetic sensor 11 in a memory unit 22 constituted from the likes of asemiconductor memory, and, based on the sensor data stored in the memoryunit 22, determines the distribution data of the sensor data, and storesthis in the memory unit 22. The control unit 121 detects thedisengagement of the wheel by comparing the sensor data and distributiondata. The control unit 121 outputs the detection result to the outputunit 13 when it detects the disengagement of the wheel.

The ROM 23 stores programs and data prestored as the factory default,and supplies the requested program or data to the control unit 121.

FIG. 10 is a block diagram showing a functional configuration example ofthe control unit 121.

The control unit 121 is constituted from a learning processing unit 131,a judgment unit 132, and a command unit 133.

The learning processing unit 131 stores the sensor data supplied fromthe magnetic sensor 11 in the memory unit 22 for a prescribed period(hereinafter referred to as the data collection period) from the startof the disengagement detection processing. The learning processing unit131, as described later with reference to FIG. 11 and FIG. 12, createsdistribution data based on the sensor data stored in the memory unit 22at the point when the data collection period elapses from the start ofdisengagement detection processing, and supplies this to the judgmentunit 132.

The judgment unit 132 stores the sensor data supplied from the magneticsensor 11 and the distribution data supplied from the learningprocessing unit 131 in the memory unit 22 after the data collectionperiod elapses from the start of disengagement detection processing. Thejudgment unit 132, as described later with reference to FIG. 11 and FIG.12, judges whether the wheel has been disengaged by comparing the valueof sensor data and distribution data.

The judgment unit 132, upon judging that the wheel has been disengaged,outputs the detection result thereof to the output unit 13. Further, thejudgment unit 132 has a built-in timer (not shown) for measuring theabnormality duration.

The command unit 133 acquires a command from the user via the input unit114, and controls the learning processing unit 131 and the judgment unit132 based on such command. The command unit 133 manages the distributiondata generation flag representing whether the distribution data has beencreated, and the learning processing unit 131 and judgment unit 132switch and execute the processing before the creation of distributiondata and the processing after the creation of distribution data based onthe value of such flag.

Next, the processing executed with the disengagement detection device101 is explained with reference to the flowcharts of FIG. 11 and FIG.12. Incidentally, this processing is started when the user commands thestart of the disengagement detection processing via the input unit 114.

At step 51, the initialization processing is executed. Specifically, thecommand unit 133 turns OFF the distribution data generation flagrepresenting whether the distribution data was created. Nevertheless,after the disengagement detection processing is temporarily stopped, andthe disengagement detection processing is resumed, when the user doesnot command the renewal of the distribution data via the input unit 114(when using the distribution data created in the previous disengagementdetection processing without change), the distribution data generationflag will be turned ON. In the following explanation, a case isexplained where the distribution data generation flag is turned OFF atstep S51.

At step S52, when the distribution data generation flag is OFF, thelearning processing unit 131 acquires the sensor data based on themagnetism in the wheel vicinity detected with the magnetic sensor 11from the magnetic sensor 11, and stores this in the memory unit 22. Whenthe distribution data generation flag is ON, the judgment unit 132acquires the sensor data based on the magnetism in the wheel vicinitydetected with the magnetic sensor 11 from the magnetic sensor 11, andstores this in the memory unit 22. Here, since the distribution datageneration flag is turned OFF in the initialization processing at stepS1, the learning processing unit 131 acquires the sensor data from themagnetic sensor 11, and stores this in the memory unit 22.

At step S53, the command unit 133 judges whether the distribution datageneration flag is ON. Here, since the distribution data generation flagis turned OFF in the initialization processing at step S1, theprocessing proceeds to step S54.

At step S54, the learning processing unit 131 judges whether the datacollection period (e.g., 1 day) has elapsed from the start ofdisengagement detection processing. Here, since the disengagementdetection processing has just started, it is judged that the datacollection period has not elapsed from the start of disengagementdetection processing, and the processing of steps S55 and S56 isskipped, and the processing proceeds to step S57.

At step S57, the command unit 133 judges whether to end thedisengagement detection processing; that is, whether the user commandedthe end of the disengagement detection processing via the input unit114. When the user commands the end of the disengagement detectionprocessing, the disengagement detection processing is ended, and, whenthe user did not command the end of the disengagement detectionprocessing, the processing returns to step S52. In the followingexplanation, unless specifically indicated, a case is explained wherethe user did not command the end of the disengagement detectionprocessing at step S57, and the processing returns to step S52.

Thereafter, at step S54, until it is judged that the data collectionperiod has elapsed from the start of disengagement detection processing,the foregoing processing steps are repeated. In other words, the sensordata output from the magnetic sensor 11 during the data collectionperiod in various situations of the vehicle, such as while running orwhen it is parked, is stored in the memory unit 22.

At step S54, when it is judged that the data collection period haselapsed from the start of disengagement detection processing, theprocessing proceeds to step S55.

At step S55, the learning processing unit 131 creates distribution databased on the sensor data stored in the memory unit 22 during the datacollection period from the start of disengagement detection processing,and supplies this to the judgment unit 132. The judgment unit 132 storesthe supplied distribution data in the memory unit 22.

Distribution data is data representing the distribution of thecombination of the biaxial values of the x axis and y axis of the sensordata output from the magnetic sensor 11 during the data collectionperiod. The learning processing unit 131 divides the range of the valuethat may be taken by the sensor data into a prescribed unit (e.g., 10milligauss) with respect to both axes of the x axis and y axis, anddetermines the quantity of sensor data having the value contained insuch range from the sensor data stored in the memory unit 22 for eachcombination of such divided range of x axis and y axis (e.g., x axis ina range of 0 to 10 milligauss and y axis in a range of 0 to 10milligauss). The learning processing unit 131 supplies, as thedistribution data of the value of sensor data, the quantity of sensordata contained in such divided range to the judgment unit 132. In otherwords, distribution data is data for representing the quantity of sensordata containing in the respective grids upon partitioning the graphillustrated in FIG. 5 or FIG. 6 into a prescribed range (unit) of a gridshape with a perpendicular line in relation to the respective axes ofthe x axis and y axis.

The sensor data output from the magnetic sensor 11 periodically changesas shown with the curve 51 of FIG. 5 (or curve 61 of FIG. 6) pursuant tothe rotation of the tire (wheel), for example, the value of sensor datastored in the memory unit 22 during the data collection period will alsobecome a distribution close to the curve shown with the curve 51 (orcurve 61). Therefore, the distribution data will become data of a valuein which the quantity of sensor data containing the grids (range) aboveand near the curve 51 (or curve 61) will be numerous, and the quantityof sensor data contained in the grids (range) apart from the curve 51(or curve 61) is 0 or close to 0.

At step S56, the command unit 133 turns ON the distribution datageneration flag representing the creation of distribution data.Thereafter, the processing proceeds to step S57, and then returns tostep S52.

After the creation of distribution data, pursuant to the processing atstep S52, after the judgment unit 132 acquires the sensor data from themagnetic sensor 11, at step S53, it is judged that the distribution datageneration flag is ON, and the processing proceeds to step S58.

At step S58, the judgment unit 132 compares the value of sensor dataacquired with the processing at step S52 and the distribution datastored in the memory unit 22. The judgment unit 132 judges that thesensor data is of a standard value when the value of sensor data is avalue of a range containing a prescribed quantity (e.g., 5 or more) ofsensor data on the distribution data, and judges that the sensor data isan abnormal value when it is a value of a range containing sensor datathat is less than a prescribed quantity. In other words, the standardvalue is a value in a range that is constantly being output from themagnetic sensor 11 during the data collection period, and, for instance,is the value in a range above and near the curve 51 of FIG. 5 (or curve61 of FIG. 6). Meanwhile, an abnormal value is a value in a range thatis not output at all or hardly output from the magnetic sensor 11 duringthe data collection period, and, for instance, includes the value of thepoint 52 (or point 62) apart from the curve 51 (or curve 61).

At step S59, the judgment unit 132 judges whether the value of sensordata is a standard value based on the results of the processing at stepS58.

At step S59, when it is judged that the value of sensor data is astandard value, the processing proceeds to step S65, and the judgmentunit 132, while the abnormality duration is being measured, stops thetimer, and stops the measurement of the abnormality duration.Thereafter, the processing proceeds to step S57, then returns to stepS52, and the subsequent processing steps are repeated.

At step S59, when it is judged that the value of sensor data is not astandard value (that it is an abnormal value), the processing proceedsto step S60.

At step S60, the judgment unit 132 judges whether the abnormalityduration is being measured. When it is judged that the abnormalityduration is not being measured; that is, when the sensor data that waspreviously a standard value becomes an abnormal value, the processingproceeds to step S64, and the judgment unit 132 activates the timer, andstarts the measurement of the abnormality duration. The judgment unit132, based on this abnormality duration, measures the time in which thevalue of sensor data continues to be in the state of an abnormal value.Thereafter, the processing proceeds to step S57, thereafter returns tostep S52, and the subsequent processing steps are repeated.

At step S60, when it is judged that the abnormality duration is beingmeasured; that is, when the sensor data of abnormal value from themagnetic sensor 11 is continuously output, the processing proceeds tostep S61.

At step S61, the judgment unit 132 judges whether the abnormalityduration exceeds the disengagement detection period. When it is judgedthat the abnormality duration is not exceeding the disengagementdetection period; that is, when the period in which the sensor data ofabnormal value from the magnetic sensor 11 being continuously output isnot exceeding the disengagement detection period, processing of stepsS62 and S63 is skipped, the processing thereafter proceeds to step S57,returns to step S52, and the subsequent processing steps are repeated.

At step S61, when it is judged that the abnormality duration hasexceeded the disengagement detection period; that is, when the period inwhich the sensor data of abnormal value from the magnetic sensor 11being continuously output is exceeding the disengagement detectionperiod, for instance, when it is judged that the magnetism in the wheelvicinity continues to be in a state of a value (e.g., the valuerepresented with the point 52 in FIG. 5 or point 62 in FIG. 6) that isdifferent from the distribution in a state where the wheel is mounted asa result of the wheel being disengaged, the processing proceeds to stepS62.

At step S62, the judgment unit 132 outputs a disengagement notificationsignal outside the disengagement detection device 1 via the output unit13 (for notifying the user that the wheel has been stolen).

At step S63, the judgment unit 132 stops the timer, and stops themeasurement of the abnormality duration. Thereafter, the processingproceeds to step S57, then returns to step S52, and the subsequentprocessing steps are repeated.

As described above, the disengagement of the wheel is detected bydetecting that the magnetism in the wheel vicinity became a valuedifferent than the distribution in a state where the wheel is mounteddue to the disengagement of the wheel based on the sensor data outputfrom the magnetic sensor 11.

Further, the disengagement detection device may combine the foregoingfluctuation detection system and distribution data detection system andemploy a combination detection system for detecting the disengagement ofa wheel. As a result of combining the two systems, the disengagementdetection device will be able to detect the disengagement of wheels withgreater precision.

Incidentally, the configuration example of the disengagement detectiondevice employing the combination detection system is the same as theconfiguration example of the disengagement detection device 101 shown inFIG. 9, and the explanation thereof is omitted.

The disengagement detection processing of the combination detectionsystem to be executed with the disengagement detection device 101 is nowexplained with reference to the flowcharts of FIG. 13 and FIG. 14.Incidentally, the distribution data generation processing to be executedat steps S101 to S107 in the combination detection system is the same asthe distribution data generation processing to be executed at steps S51to S57 of FIG. 11 in the distribution data detection system, and theexplanation thereof is omitted. The processing to be executed by thedisengagement detection device 101 after the distribution data iscreated is explained. Further, in the following explanation, unlessspecifically indicated, a case is explained where the user did notcommand the end of the disengagement detection processing at step S107corresponding to the processing at S57 of FIG. 11, and the processingreturns to step S102.

At step S105, after the distribution data is created, at step S106, thedistribution data generation flag is turned ON, and the processingproceeds to step S107, and then returns to step S102.

At step S102, the judgment unit 132 acquires the sensor data based onthe magnetism in the wheel vicinity detected with the magnetic sensor 11from the magnetic sensor 11, and stores this in the memory unit 22.

At step S103, the command unit 133 judges whether the distribution datageneration flag is turned ON. Here, since the distribution datageneration flag is turned ON in the processing at step S106, theprocessing proceeds to step S108.

At step S108, the judgment unit 132 compares the value of sensor dataacquired with the processing at step S102 and the distribution datastored in the memory unit 22 with the processing similar to step S58 ofFIG. 11 in the foregoing distribution data detection system.

At step S109, the judgment unit 132 judges whether the value of sensordata is a standard value based on the results of the processing at stepS108.

At step S109, when it is judged that the value of sensor data is astandard value, the processing proceeds to step S115, and the judgmentunit 132, when the abnormality duration is being measured, stops thetimer, and stops the measurement of the abnormality duration.Thereafter, the processing proceeds to step S107, then returns to stepS102, and the subsequent processing steps are repeated.

At step S109, when it is judged that the value of sensor data is not astandard value (that it is an abnormal value), the processing proceedsto step S110.

At step S110, the judgment unit 132 compares the latest sensor data andthe previous sensor data stored in the memory unit 22 with theprocessing similar to step S12 of FIG. 8 in the fluctuation detectionsystem, and judges whether the value of sensor data of at least onedirection among the x axis direction and y axis direction has fluctuatedby the fluctuation threshold value (0.8×|M1−M2|) or more. When it isjudged that the value of sensor data of at least one direction among thex axis direction and y axis direction has fluctuated by the fluctuationthreshold value or more, the processing proceeds to step S114.

Nevertheless, at step S110, unlike the processing of step S12 of FIG. 8in the fluctuation detection system, the variation of sensor data is notsought based on the average value of the sensor data, it is sought bycomparing the values of each sensor data. This is because thedistribution data is not created based on the average value of thesensor data, but rather based on each sensor data, and, since thecomparison of the sensor data and distribution data at step S108 isperformed against each sensor data, the processing at step S110 is alsoperformed based on the variation of each sensor data pursuant thereto.

At step S114, the judgment unit 132 activates the timer, and starts themeasurement of the abnormality duration. The judgment unit 132, based onthis abnormality duration, measures the period in which the variation ofsensor data, while remaining an abnormal value, continues to be in astable state (M2±B) less than the fluctuation threshold value; that is,the time when the magnetism in the wheel vicinity stabilizes at theabnormal value M2. Thereafter, the processing proceeds to step S107,then returns to step S102, and the subsequent processing steps arerepeated.

At step S110, when it is judged that neither value of sensor data ofboth directions of the x axis direction and y axis direction isfluctuating by the fluctuation threshold value or more, the processingproceeds to step S111.

At step S111, the judgment unit 132 judges whether the abnormalityduration exceeded the prescribed disengagement detection period. Whenthe measurement of the abnormality duration has not started, or when thedisengagement detection period has not elapsed from the point when themeasurement of abnormality duration was started, it is judged that theabnormality duration has not exceeded the disengagement detectionperiod, and the processing at steps S112 to S113 is skipped, theprocessing proceeds to step S107, thereafter returns to step S102, andthe subsequent processing steps are repeated.

At step S111, when it is judged that the abnormality duration exceededthe disengagement detection period; that is, when the value V of thesensor data of at least one direction among the x axis direction and yaxis direction fluctuates by the fluctuation threshold value(0.8×|M1−M2|) or more, the value of sensor data becomes an abnormalvalue, and, while remaining at the abnormal value, and a stable periodin which the fluctuation range of the average value is less than thefluctuation threshold value (M2±B) continues during the disengagementdetection period, for instance, as shown with section T2 after sectionT0 in FIG. 4, the magnetism in the wheel vicinity will change from valueM1 to value M2 (value represented with the point 52 of FIG. 5 or point62 of FIG. 6), which is different from the distribution in a state wherethe wheel is mounted, due to the disengagement of the wheel, and, whenit is judges as being stable at value M2, the processing proceeds tostep S112.

In other words, in comparison to the distribution data detection systemwhich judged the disengagement of the wheel when the sensor data in anabnormal value continues during the disengagement detection periodregardless of whether the value of sensor data is stable, thecombination system judges the disengagement of the wheel when the valueof sensor data fluctuates by the fluctuation threshold value or more andbecomes an abnormal value, and such value in a stable state continuesduring the disengagement detection period. Therefore, a more accuratedetection is possible.

At step S112, the judgment unit 132 outputs a disengagement notificationsignal outside the disengagement detection device 101 via the outputunit 13.

At step S113, the judgment unit 132 stops the timer, and stops themeasurement of the abnormality duration. Thereafter, the processingproceeds to step S107, then returns to step S102, and the subsequentprocessing steps are repeated.

Incidentally, during the measurement of the abnormality duration, atstep S110, when it is judged that the average value of the sensordirection of at least one direction among the x axis direction and yaxis direction fluctuated by the fluctuation threshold value or more;that is, when the value of sensor data is fluctuating unstably in shortcycles in an abnormal value, it is judged that the magnetism in thewheel vicinity is unstable due to reasons other than the disengagementof the wheel, and the processing proceeds to step S114, the timer isreset, and the remeasurement of the abnormality duration is started.

As described above, the magnetism in the wheel vicinity changingsignificantly in comparison to the stationary state due to thedisengagement of the wheel based on the sensor data output from themagnetic sensor 11, and the disengagement of the wheel is detected bydetecting that such changed state became stable.

1. A disengagement detection device for detecting the disengagement of awheel from a vehicle, comprising: detection means for detecting themagnetism inside said vehicle at or near said wheel mounted on saidvehicle; and judgment means for judging the disengagement of said wheelfrom said vehicle based on whether the value of said magnetism detectedwith said detection means changed by a prescribed threshold value ormore, and whether a first stable state centered around a first valuechanged to a second stable state centered around a second value.
 2. Adisengagement detection device according to claim 1, wherein saiddetection means detects magnetisms in a plurality of differentdirections, and said judgment means judges the disengagement of saidwheel from said vehicle based on the comparison of the value of saidmagnetisms detected with said detection means, and the distribution ofthe value of the magnetisms detected previously with said detectionmeans in a state where said wheel is mounted on said vehicle.
 3. Adisengagement detection device for detecting the disengagement of awheel from a vehicle, comprising: detection means for detectingmagnetisms in a plurality of different directions inside said vehicle ator near said wheel mounted on said vehicle; and judgment means forjudging the disengagement of said wheel from said vehicle based on thecomparison of the value of said magnetisms detected with said detectionmeans, and the distribution of the value of the magnetisms detectedpreviously with said detection means in a state where said wheel ismounted on said vehicle.
 4. A disengagement detection device accordingto claim 1, further comprising notification means for notifying that thedisengagement of said wheel from said vehicle has been detected withsaid judgment means.
 5. A disengagement detection device for detectingthe disengagement of a wheel from a vehicle, comprising: a detectionstep for detecting the magnetism inside said vehicle at or near saidwheel mounted on said vehicle; and a judgment step for judging thedisengagement of said wheel from said vehicle based on whether the valueof said magnetism detected by the processing in said detection stepchanged by a prescribed threshold value or more, and whether a firststable state centered around a first value changed to a second stablestate centered around a second value.
 6. A disengagement detectiondevice for detecting the disengagement of a wheel from a vehicle,comprising: a detection step for detecting magnetisms in a plurality ofdifferent directions inside said vehicle at or near said wheel mountedon said vehicle; and a judgment step for judging the disengagement ofsaid wheel from said vehicle based on the comparison of the value ofsaid magnetisms detected by the processing in said detection step, andthe distribution of the value of the magnetisms detected previously bythe processing in said detection step in a state where said wheel ismounted on said vehicle.